If they're not blood related and didn't live for years under the same roof while being underage, then no, it's not incest. For it to be considered anywhere close to incest, the step siblings need to have lived as "family" at least for a while
incest only counts if they are blood related or if u lived with your "step sibling" since u were young like u've been living with them for 10+ years and u see each other as "real siblings" . But if u only met them for few months and already an adult i thinks that's not incest anymore.
Guys i have a question. Does incest only happens on blood related or not? Is it legal to do it with your step family?